The present invention relates to a display panel having an image display function, and a printed circuit board mounted on the display panel.
Owing to the recent advancement in ultrafine processing technology, liquid crystal material technology, and mounting technology, liquid crystal panels capable of displaying various images such as television images at a practically satisfactory level in diagonal size of 5 to 50 cm are presented on a commercial base.
Besides, color display is easily realized by forming an RGB coloring layer in one of the two glass substrates for composing a liquid crystal panel.
In particular, in a so-called active type liquid crystal panel incorporating switching elements in every pixel, an image of high contrast ratio is obtained in high speed response with less crosstalk.
Such liquid crystal panels are generally composed in a matrix comprising 100 to 1000 scanning lines and 200 to 2000 signal lines, and lately both large screen and high definition are promoted simultaneously in relation to the increase in the display capacity.
FIG. 6 is a perspective view of an active type liquid crystal panel, in which the active type liquid crystal panel is composed of an active substrate 2, a confronting counter substrate 9, and liquid crystal filled between them.
The active substrate 2 has a plurality of scanning lines formed on a principal surface of a transparent insulating substrate, and a plurality of signal lines substantially orthogonal to the scanning lines formed at least across one insulating layer, and at least one switching element and at least one pixel electrode are disposed at every intersection of scanning line and signal line, and a terminal electrode group of the scanning lines and signal lines is disposed outside of the image display region.
The counter substrate 9 is composed of a glass substrate which is a transparent insulating substrate having a transparent conductive counter electrode.
The active substrate 2 and counter substrate 9 are formed across a specific distance of several microns by way of a spacer such as resin fiber or beads, and their gap is a closed space sealed by sealing material and filling material made of organic resin on the peripheral edge of the counter substrate 9, and this closed space is filled with liquid crystal.
In the case of color display, the closed space side of the counter substrate 9 is coated with a coloring layer containing either a dye or a pigment or both, as an organic thin film of about 1 to 2 xcexcm in thickness, so that a color display function is provided. In this case, the glass substrate 9 is also called a color filter.
Depending on the properties of the liquid crystal material, a polarizing plate is adhered to either the upper surface of the counter substrate 9 or the lower surface of the glass substrate 2, or on both sides, so that the liquid crystal panel 1 functions as an electro-optical element.
In the majority of panels, at the present, the Twisted Nematic (TN) system is used in the liquid crystal, and it requires two polarizing plates.
In thus constituted liquid crystal panel, outside the image display region of the active substrate 2, for example, on the terminal electrode group 6 of scanning lines, a semiconductor integrated circuit chip 3 for supplying drive signals is mounted by direct connecting Chip-On-Glass (COG) system, whereas on the terminal electrode group 5 of signal lines, a Tape-Carrier-Package (TCP) film 4 is mounted by the TCP system of connecting and fixing with a conductive adhesive.
The TCP film 4 has a terminal of copper foil (not shown) of gold plating or solder plating, for example, formed on a polyimide resin thin film of about 0.1 mm in thickness.
Two mounting methods are shown herein, but actually either method is properly selected.
The image display unit of the liquid crystal panel 1, and the terminal electrode groups 5, 6 of signal lines and scanning lines are connected by means of wiring lines 7, 8, but the wiring lines 7, 8 are not always required to be composed of same conductive material as the terminal electrode groups 5, 6.
In this liquid crystal panel, liquid crystal cells are composed of transparent conductive pixel electrodes formed on the active substrate 2, transparent conductive counter electrodes formed on the counter substrate 9, and liquid crystal filled between the two glass substrates.
In the recently developed In-Plain-Switching (IPS ) type liquid crystal panel capable of expanding the viewing field angle, the liquid crystal cells are composed of a pair of comb electrodes formed on one glass substrate (active substrate) and the liquid crystal filled between the two glass substrates, and hence the transparent electrode (counter electrode) is not needed on the color filter, but the detail is omitted herein.
To display an image on the liquid crystal panel, an electric signal must be given to the terminal electrodes of the scanning lines and signal lines by TCP or COG mounting as mentioned above. Recently, to save the mounting cost, or to enhance the mounting reliability by decreasing the number of connecting positions, the COG mounting tends to be preferred.
FIG. 7 is a magnified view of essential parts by COG mounting of a driving integrated circuit chip 3 on the active substrate 2, and in the COG mounting, since systems of power source line, input signal line, and clock line scanning to a plurality of driving integrated circuit chips cannot be supplied in each chip as in the TCP mounting, it is needed to form about 20 to 40 conductive bus lines 10 on the peripheral edge of the active substrate 2, but in the digitized signal system, in order to be applicable to high image quality, henceforth, the number of bits tends to increase rapidly.
To form active elements, it is also needed to form scanning lines and signal lines, and these wiring lines can be formed simultaneously with the active elements, which is more rational.
Forming places of wiring lines 10 can be easily insulated by forming an insulation layer on the surface, and, for example, by utilizing the space effectively by disposing wiring lines 10 beneath the integrated circuit chip 3, it may contribute to narrowing of peripheral margin of the liquid crystal panel.
In the COG mounting, however, in the screen diagonal size of 6 (15 cm) or more at which the resistance value of the wiring line 10 is high, as shown in FIG. 8, it is required to use together with a bus flexible film 11.
The bus flexible film 11 is similar to the TCP film 4, that is, a wiring line 12 made of a conductive thick film is formed on a flexible film. Its length is as long as 20 to 50 cm, and the wiring line connecting between parallel lines crosses orthogonally with the parallel lines, and hence it requires several or more multiple layers, which is very expensive, and it is difficult to lower the cost.
Inevitably, however, the bus flexible film 11 is used as explained below.
When manufacturing a active liquid crystal panel, since the film thickness of the conductive lines such as scanning lines and signal lines is about 0.5 xcexcm, the resistance value of the conductive lines cannot be lowered sufficiently, and the resistance of the wiring lines 10 increases due to the shape effect (ratio of length/width of wiring pattern), and a sufficient current cannot be passed by the bus wiring lines 10 alone. More specifically, for example, by using an aluminum film of about 0.3 xcexcm in thickness in the wiring line material, if a surface resistance of 0.1 xcexa9/cm2 is obtained, in the case of wiring width of 50 xcexcm and wiring length of 25 xcexcm, the resistance value of the wiring lines reaches as much as 500xcexa9, and it is hence difficult to use in the power source lines of mA unit or higher.
Similarly, in the input signal and clock lines, such wiring resistance is added to the input resistance of the semiconductor integrated circuit, and in the case of high speed transfer of signal (more than hundreds of kHz), the signal waveform becomes rounded off, and the semiconductor integrated circuit may fail to work.
The invention is to solve these conventional problems, and is intended to supply necessary signals and power source to the mounted semiconductor integrated circuit chip through wiring lines having a low resistance.
In the invention, an opening or a recess is formed in the mounting position of the driving integrated circuit chip on the printed circuit board which is mounted outside of the image display area of the active substrate, and the wiring lines of low resistance formed on the printed circuit board are used as bus lines.
Therefore, according to the invention, since the resistance value of the bus lines connected to the driving chip can be decreased, the signals can be transmitted at high speed without deforming the signal waveform.
Also according to the invention, wiring lines of low resistance value composed of a thick conductive material formed on the printed circuit board can be used as the bus lines such as power source lines, input signal lines and clock lines, and it is not necessary to use the conventional long bus flexible film.
Further according to the invention, not only the printed circuit board can be mounted on the active substrate, but also the cost is reduced.
Moreover, according to the invention, in a display panel of a particularly large screen size, the printed circuit board can be easily mounted on the active substrate.